(1) Field of the Invention
The present invention relates to a structure of ohmic electrodes on compound semiconductors.
(2) Description of the Prior Art
In the prior art of preparing compound semiconductor devices, formation of the ohmic contacts on n-GaAs as a compound semiconductor is performed by epitaxially growing an In.sub.x Ga.sub.1-x As layer (0&lt;x.ltoreq.1) on the n-GaAs and depositing in layers a metal or an alloy to form electrodes. FIGS. 1 and 2 show prior art examples of ohmic electrode structures.
In a case of the conventional example shown in FIG. 1, an In.sub.x Ga.sub.1-x As layer 20 is formed on a compound semiconductor (n-GaAs) 10, and there is provided on the top of the layers a metal layer 40 consisting of a Ti layer 41, a Pt layer 42 and an Au layer 43.
On the other hand, in a case of the conventional example shown in FIG. 2, there is formed on a compound semidoncuctor (n-GaAs) 10 an In.sub.x Ga.sub.1-x As layer 20, on which a barrier layer 35 consisting of tungsten silicide (WSi) is layered. In addition, a metal layer 40 consisting of a Ti layer 41, a Pt layer 42 and an Au layer 43 are provided on the top of the layers in order to form electrodes. More clearly, contrasting the structure with that shown in FIG. 1, this second structure is constructed such that tungsten silicide barrier layer 35 is provided between In.sub.x Ga.sub.1-x As layer 20 and metal layer 40 (see Japanese Patent Application Laid-Open Sho 63 No.276267).
In the case of the conventional example shown in FIG. 1, metal layer 40 (Ti/Pt/Au) forming an electrode is liable to react with In.sub.x Ga.sub.1-x As layer 20. More specifically, the metal (Ti/Pt/Au) constituting metal layer 40 is easy to diffuse into In.sub.x Ga.sub.1-x As layer 20, and the metal (In/Ga/As) constituting In.sub.x Ga.sub.1-x As layer 20 is also subject to diffuse into metal layer 40. Therefore, this structure could not be treated at a greatly elevated temperature after the formation of the electrodes. For instance, if the layered structure was heated at 390.degree. C. for one minute, the contact resistance .rho..sub.c of the structure would be increased by about three orders from the order of 10.sup.-8 .OMEGA.cm.sup.2 to the order of 10.sup.-5 .OMEGA.cm.sup.2.
In the case of the conventional example shown in FIG. 2, because tungsten silicide barrier layer 35 is provided between In.sub.x Ga.sub.1-x As layer 20 and metal layer 40 (Ti/Pt/Au), the structure is stable presenting no increase in its contact resistance even if it is subjected to heat treatment of 400.degree. C. or more. Nevertheless, the composition control of the barrier layer is difficult to achieve since the sputtering efficiencies (the WSi (tungsten silicide) is formed by sputtering.) are different between W and Si. Further, it is impossible to increase the purity of WSi to a great extent since WSi is a sintered body. There is also another drawback that the workablity is low since WSi generally assumes a columnar polycrystalline structure.